1. Field of the Invention
The present invention relates to an image forming system, such as a copier or printer of an electrophotographic type or an electrostatic storage type.
2. Description of the Related Art
Among image forming apparatuses, such as copiers and printers of an electrophotographic type and of an electrostatic storage type, image forming apparatuses wherein a sheet feeding unit is located at the lower portion of the main body have long been known. Further, image forming apparatuses are also known wherein the image forming processing speed can be changed in accordance with the type (the sheet type) of the sheets supplied for image forming (see Japanese Patent Laid-Open Publication No. 2000-305438, corresponding to U.S. Pat. No. 6,301,452).
The sheet conveying operation of an image forming apparatus in FIGS. 13A to 13C, for example, will be explained while referring to FIG. 14. A controller transmits, to an engine controller, a print reservation command that designates a main body sheet feeding unit 20a and a sheet type detection mode (401), and thereafter, transmits a print start command (402). Upon receiving both commands, the engine controller prepares for printing (403), and initiates the feeding of sheets from the main body sheet feeding unit 20a (404, FIG. 13A). When a sheet fed in this manner has reached a sheet detection position (405, FIG. 13B), the engine controller temporarily halts the conveying of the sheet (406), and detects the sheet type (407).
When the sheet type has been identified, the engine controller switches to an image forming condition consonant with the sheet type that was identified (408), and resumes sheet conveying (409). Thereafter, the engine controller outputs a vertical sync signal (a /TOP signal) to permit the controller to output image data (410, FIG. 13C).
When the sheets employed for this image forming apparatus are plain paper, for example, the feeding speed for registration rollers 15 is set equal to the conveying speed of cassette convey rollers 22a and intermediate convey rollers 23a. When the sheets are thick paper, for example, the feeding speed for the registration rollers 15 is set lower than the conveying speed of the cassette convey rollers 22a and the intermediate convey rollers 23a. 
Further, another image forming system is also known wherein, as shown in FIGS. 15A to 15D, an external sheet feeding apparatus 20b is attached to the bottom of the main body sheet feeding unit 20a. The sheet conveying operation of this image forming system will now be described while referring to FIG. 16. When printing preparation is completed, sheets stacked in the external sheet feeding apparatus 20b are separated by a cassette pickup roller 21b (601, 602; FIG. 15A), and each sheet is conveyed to a merging point 24 by cassette convey rollers 22b and intermediate convey rollers 23b (603 to 605; FIG. 15B). Then, the sheet is conveyed by intermediate convey rollers 23a, as well as by the cassette convey rollers 22b and the intermediate convey rollers 23b, and when the leading edge of the sheet is detected by a registration sensor 11, sheet conveying is temporarily halted (606 to 610; FIG. 15C).
In this temporary halted state, the type of sheet is detected by a sheet type detection sensor 12, which is located between the intermediate convey rollers 23b and the registration rollers 15 (611). When the sheet type has been identified, the image forming condition is changed to one in consonance with the identified sheet type (612).
At a predetermined timing, the sheet begins to be conveyed to a transfer conveying belt 9 by the registration rollers 15 and the intermediate convey rollers 23a and 23b (613 and 614), and a vertical sync signal (a /TOP signal) is output to begin the output of image data (615).
When sheets are plain paper, for example, the sheet conveying speed for the registration rollers 15 and the intermediate convey rollers 23a equals the sheet conveying speed for the intermediate convey rollers 23b, and problems, such as the jamming of sheets, do not occur.
However, when sheets are thick paper, for example, the conveying speed for the registration rollers 15 and the intermediate convey rollers 23a is lower than the conveying speed for the intermediate convey rollers 23b, i.e., the conveying speeds are not equal, the jamming of sheets occurs (616; FIG. 15D).
Accordingly, needed is an improved image forming system and a control method therefore, and a control unit for an image forming apparatus.
Also needed is an image forming system wherein a recording material, supplied from an externally connected feeding unit, can also be normally conveyed, a control method therefor, and a control unit for an image forming apparatus.
Also needed is an image forming system wherein a recording material to which an image has been fixed can also be normally conveyed to an externally connected sheet delivery apparatus, a control method therefor, and a control unit for an image forming apparatus.